When a piezoelectric single-crystal device is a rectangular parallelepiped, for example, as shown in FIG. 2 and the longitudinal direction is the polarization direction 3 (hereinafter the polarization direction is referred to as PD), the magnitude of vibration in the polarization direction PD (vibration in the longitudinal direction) when a voltage is applied in the polarization direction PD is represented by an electromechanical coupling factor k33 in a longitudinal vibration mode that is proportional to the square root of the conversion efficiency of the electric energy and the mechanical energy. The greater k33 value is, the more the conversion efficiency improves. When the piezoelectric single-crystal device is plate-shaped, for example, as shown in FIG. 3, the conversion efficiency increases with an electromechanical coupling factor k31 in the direction 1 (lateral vibration mode) orthogonal to the polarization direction PD. The shape of the piezoelectric single-crystal device may be a bar, a square plate, or a circular plate, in addition to the above-mentioned rectangular parallelepiped and plate-shape, and the electromechanical coupling factor can be similarly determined for each differently shaped device.
With respect to the piezoelectric single-crystal device, Japanese Unexamined Patent Application Publication No. 6-38963 discloses an ultrasonic wave probe using a piezoelectric device formed of a solid solution single crystal of lead zinc niobate-lead titanate (PZN-PT), for example. It is described that the electromechanical coupling factor (k33) in the polarization direction of such a piezoelectric single-crystal device is 80 to 85% which is sufficiently large to obtain a probe with excellent sensitivity.
However, an ingot and a wafer (substrate) formed of a piezoelectric single crystal of lead zinc niobate-lead titanate (PZN-PT) is expensive compared with those formed of lead zirconium titanate (Pb(Zr, Ti)O3: PZT) which is a conventional piezoelectric device material. Additionally, a piezoelectric single crystal formed of lead magnesium niobate-lead titanate (PMN-PT) using lead magnesium niobate (PMN) instead of lead zinc niobate (PZN) and other piezoelectric single crystals having a similar composition are also expensive.
The causes that the ingots and wafers formed of such piezoelectric single crystals are expensive are thought to be the following:
A first cause is evaporation of lead oxide (PbO) during the single-crystal growth. In the Melt Bridgman method, a single crystal is grown by melting a powder, calcined material, or sintered compact of raw material components for forming the piezoelectric single crystal and solidifying it in one direction. In the Solution Bridgman method, a single crystal is grown by dissolving the raw material components for forming the piezoelectric single crystal in a solution by using a flux and solidifying it in one direction. In single crystal growth by the Melt Bridgman method or the Solution Bridgman method, since the vapor pressure of lead oxide (PbO) existing as a component or the flux becomes high at a temperature used for the single crystal growth, the lead oxide significantly evaporates. Consequently, during the solidification process, a pyrochlore phase which deviates from a desired composition ratio and has a poor piezoelectric characteristic is precipitated or a large number of microcrystals having irregular crystal orientations are precipitated on the pyrochlore phase. Therefore, a crystal yield and a wafer yield are significantly reduced. Here, the term “crystal yield” means percentage (%) of mass of a complete single-crystal portion not having the pyrochlore phase and any heat cracks to the total mass of the fed raw material. The term “wafer yield” means percentage (%) of the number of complete wafers not having the pyrochlore phase and any heat cracks to the total number of the wafers prepared by cutting the resulting single-crystal portion so as to have a desired orientation and a desired thickness with a cutting tool such as a wire saw.
A second cause is the occurrence of cracks during the single-crystal growth. When the single crystal is grown by the Melt Bridgman method or the Solution Bridgman method, a temperature difference is generated in the growth direction of the crystal in a crucible and a temperature difference is generated between the central portion of the single crystal and the external surface of the single crystal being in contact with the inner wall of the crucible in a cooling process to a room temperature during and after the growth of the single crystal. Cracking of the single crystal (heat cracking) tends to occur during the growth process and the cooling process due to thermal strain caused by these temperature differences. Therefore, the crystal yield and the wafer yield are significantly decreased. Additionally, there is a tendency that the cracking occurs more readily in a single crystal having better crystallinity.
A third cause is the occurrence of chipping during the manufacturing of a piezoelectric single-crystal device. After the single crystal growth by the Melt Bridgman method or the Solution Bridgman method, an ingot of the resulting piezoelectric single crystal is cut into wafers and single-crystal plates having a desired piezoelectric device shape. During the cutting process, fine chipping occurs at the peripheries of the cut end faces of the single-crystal plate. Therefore, the single-crystal plate yield of the piezoelectric single-crystal device is significantly decreased. Here, the term “single-crystal plate yield” means the percentage (%) of the number of complete single-crystal plates not having chipping to the total number of the single-crystal plates having a desired size obtained by cutting the resulting wafer with a cutting tool such as a dicing saw.
Thus, when the piezoelectric single-crystal plate and the piezoelectric single-crystal device are manufactured by growing the piezoelectric single crystal using a powder, calcined material, or sintered compact of the raw material components for forming the piezoelectric single crystal, the decreases in the crystal yield and the wafer yield of the piezoelectric single-crystal device, and the single-crystal plate yield cannot be avoided and consequently the production cost is increased. As a result, there have been negative effects such as the number of fields in which they can be applied is limited.